Conventionally, in image forming apparatus, such as a copying machine, a density correction process has been carried out to read-in image data so as to conform (i) a density of a printed image that is actually printed out with (ii) a density of image data of a document that is read in from a device, such as a scanner. This density correction process is generally carried out by using, for example, a method in which a quantity of correction predefined based upon precedently created density correction data is added/subtracted to/from the read-in image data.
Meanwhile, there is a problem that the density of the printed image that is printed out based upon the image data to which the density correction process is precedently carried out does not conform with the density of the input image (for example, document image) as a result that sensitivity of a photosensitive drum changes due to various factors, such as changes over time in sensitivity characteristic of the photosensitive drum, changes of environmental temperatures, or other factors. Therefore, the density correction data used in the density correction process have to be updated at certain timing.
An example of such density correction data updating method is disclosed in the Japanese Patent Application Publication No. 2002-335401 (published on Nov. 22, 2002) (hereinafter, referred to as published art). In this method, test patterns for tone process modes are formed in different regions on one transfer material (sheet) and are developed. Subsequently, the formed and developed test patterns are read in, and the density correction data are created based upon this read-in results.
In addition, there is another method that has been known (termed as conventionally-known-art). In this method, one test pattern is formed on a certain image bearing member, the one test pattern being for one of a plurality of tone processes that are carried out when a normal image formation motion is carried out. Then, density of this test pattern is detected. Based upon this detected density value, density correction data applicable to the above-mentioned tone process is created. Subsequently, by shifting this density correction data at a certain shifting quantity, sets of density correction data respectively applicable to the other plurality of tone processes are created.
Neither the published art nor the conventionally-known-art considers inaccuracy in measuring density, the inaccuracy caused by the tone expression of the test patterns formed on a transfer material or on an image bearing member. The test patterns are usually expressed with a tone expression expressed in the respective tone processes. In either of the arts in which test patterns expressed in such tone expression are used for creating density correction data, the image data of the read-in test patterns would possibly be inaccurate in count of dots (dot count) and in measured density. Therefore, either of the arts has a problem in that there is no confidence level in their density corrections because appropriate density correction data cannot be expected as described above. Especially, because a number of dots is extremely few in a highlighted section in test patterns, measurement in a quantity of toner adhered in the highlighted section tends to be inaccurate, and therefore the density correction data lacks confidence level in terms of the highlighted section. In other words, because the inaccuracy in the dot count and in measured density occurs significantly in the highlighted section in the read-in images of the test patterns, the confidence level of the density correction data corresponding to the highlighted section decreases further than the other section of the image.